1. Field of the Invention
This invention relates to optical metrology used in the manufacturing of semiconductor devices, and more particularly to a zoned diffraction order sorting filter for spectrometers used in semiconductor optical metrology systems, and to a method for wavelength calibration of spectrometers with zoned order sorting filters.
2. Description of Related Art
One type of optical metrology involves directing an incident beam at a structure on a workpiece, measuring the resulting diffraction signal, and analyzing the measured diffraction signal to determine various characteristics of the structure. The workpiece can be a semiconductor wafer or substrate, a photomask, or a magnetic medium. In manufacturing of workpieces, periodic gratings are typically used for quality assurance. For example, one typical use involves fabricating a periodic grating in proximity to the operating structure of a semiconductor chip. The periodic grating is then illuminated with electromagnetic radiation, and the electromagnetic radiation that deflects off of the periodic grating are collected as a diffraction signal. The diffraction signal is then analyzed to determine whether the periodic grating, and by extension whether the operating structure of the semiconductor chip, has been fabricated according to specifications.
In one conventional system, the diffraction signal collected from illuminating the periodic grating (i.e. the measured diffraction signal) is compared to a library of simulated diffraction signals. Each simulated diffraction signal in the library is associated with a hypothetical profile, i.e. cross-section, of the periodic grating. When a match is made between the measured diffraction signal and one of the simulated diffraction signals in the library, the hypothetical profile associated with the simulated diffraction signal is presumed to represent the actual profile of the periodic grating. The hypothetical profiles, which are used to generate the simulated diffraction signals, are generated based on a profile model that characterizes the structure to be examined. Thus, in order to accurately determine the profile of the structure using optical metrology, a profile model that accurately characterizes the structure should be used.
With increased requirements for throughput, decreasing size of the test structures, smaller illuminated spot sizes, and lower cost of ownership, there is greater need to optimize designs of optical metrology systems to meet these design goals. Characteristics of the optical metrology system including sampling time, range of measurement capabilities, accuracy and repeatability of diffraction signal measurements are essential to meeting these ever-increasing requirements.
Spectrometers are used to measure diffraction signals over a wide range of wavelengths, from the near-infrared (NIR), over visible light (VIS), and ultraviolet (UV), to the deep ultraviolet (DUV) parts of the electromagnetic spectrum. A spectrometer typically employs a blazed diffraction grating to disperse an optical signal onto an array detector capable of measuring the intensity of different wavelengths present in the optical signal, i.e. the diffraction signal. A diffraction grating disperses the optical signal into a multitude of diffraction orders, of which typically only the first order is used to perform the actual diffraction signal measurement and matching against a diffraction signal library. However, when the wavelength range of the diffraction signal exceeds one octave, as is typically the case in semiconductor optical metrology systems, the diffraction orders generated by the grating start to overlap each other. Due to diffraction order overlap, in addition to measuring the intensity of the first diffraction order at a calibrated wavelength λ, the same array detector location, i.e. pixel, would also see and detect the second diffraction order intensity of wavelength λ/2. In the case of very large wavelength ranges, it may also see and detect the third diffraction order intensity of wavelength λ/3, etc. To prevent the detection of unwanted higher diffraction orders at any array detector pixel, an order sorting filter (OSF) may be employed. In the simplest embodiment, an OSF is a suitable optical long-pass or band-pass filter installed in front of the array detector, which prevents shorter wavelengths of the higher diffraction orders from reaching the detector. The OSF may comprise multiple zones spanning the length of the array detector, where each zone has different passband characteristics, or it may have continuously-changing passband characteristics. A zoned OSF is simpler and relatively inexpensive compared to the continuously-variable type, and is typically used for compact and fast spectrometers.
One of the drawbacks of a zoned OSF is the appearance of anomalies in measured diffraction signals at wavelengths in the vicinity of joints between OSF zones. In a zoned OSF filter, each zone is typically implemented as a thin-film stack filter on a common optically-transparent substrate. The inventors have realized that due to varying passband characteristics of the zones, the optical length traversed by the light beam through the thin-film stack filter of each zone also, in general, varies. The sharp change of traversed optical length at the zone joint may cause unwanted dispersion of light, such that light incident at the zone joint is spread sideways onto multiple pixels of the array detector. In addition to spreading onto multiple adjacent pixels, this dispersion may cause a sideways shift of the centroid of the image projected upon the detector. The inventors believe this image spreading and shifting to be the main cause of unwanted localized anomalies in the measured diffraction signals at the zone joints. Thus, to improve accuracy of measured diffraction signals, there exists a need to minimize unwanted dispersion at zone joints in an OSF, or otherwise correct measured diffraction signals affected by dispersion at OSF zone joints.